Method to reduce peak to average power ratio in multi-carrier modulation receivers

ABSTRACT

A method for performing channel estimation in an orthogonal frequency division multiplexing (OFDM) signal includes choosing reserved tones to be part of a pilot pattern, and using the reserved tones in the pilot pattern to perform the channel estimation. An apparatus for use in performing channel estimation in an OFDM system includes a receiver configured to receive a transmitted OFDM signal; a pilot symbol extractor configured to extract pilot symbols from the OFDM signal; and a channel estimator configured to perform the channel estimation, including using reserved tones as pilot tones.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.12/415,747, filed Mar. 31, 2009, issuing as U.S. Pat. No. 8,306,148 onNov. 6, 2012, which claims the benefit of U.S. Provisional PatentApplication No. 61/116,533, filed Nov. 20, 2008, the entire contents ofwhich are incorporated herein by reference.

BACKGROUND

1. Field of the Invention

The present invention generally relates to digital video broadcasting(DVB). More specifically, the present invention relates to reducing Peakto Average Power Ratio (PAPR) in multi-carrier modulation based DVBsystems.

2. Background Art

As known to those of skill in the art, DVB is the European consortiumstandard for the broadcast transmission of digital terrestrialtelevision. DVB systems transmit a compressed digital audio/videostream, using multi-carrier modulation, such as orthogonal frequencydivision multiplexing (OFDM). Another popular method of transmittingsignals is digital video broadcasting-terrestrial (DVB-T). Whenbroadcasters employ this method, the transmitted signals do not travelvia cable. Instead, they move from aerial antenna to aerial antenna, andfrom a signal blaster to home receiver.

DVB-T broadcasters transmit data using a compressed digital audio-videostream, with the entire process based on a Moving Picture Expert Group(MPEG)-2 standard. These transmissions may include all kinds of digitalbroadcasting, including high definition television (HDTV) and otherhigh-intensity methods. This is an improvement over the old analogsignals, which required separate streams of transmission.

Impulse noise is a growing concern in DVB-T transmissions because of itsnegative impact upon transmitted digital data and is common in OFDMbased systems. Although OFDM based DVB-T system receivers provide alevel of inherent immunity against impulse noise, this immunity isinsufficient to ensure that digital TV signals are reliably transmittedin accordance with applicable standards. As an example, OFDM systemsinclude data that is transmitted on multiple carriers simultaneously. Inreal life, when data is transmitted, it is passed through a datachannel. This data channel subsequently manifests itself as amultiplicative impairment (e.g., impulse noise) on one of more of thesecarriers when they are received on a receiver side.

While impulse noise has traditionally been ignored in North American TVreceiver market, several standards/specifications in the European marketrequire high tolerance to impulse noise. Another troublesome aspect ofOFDM systems is PAPR, which impacts the presence and effects of impulsenoise. By reducing PAPR, the effects of impulse noise maycorrespondingly be reduced.

Some of the traditional techniques for reducing PAPR in OFDM systemsinclude, by way of example, tone reservation, tone injection, oradaptive constellation extension. Each of these techniques is wellunderstood by those of skill in the relevant art and will not bediscussed in depth herein. All of these techniques, however, causeeither a loss in data rate or an increase in the signal-to-noise ratio(SNR) required for reception. These techniques, therefore, have onlylimited utility.

To assist a receiver in overcoming multi-path distortion, pilot signalswith known data patterns are transmitted. The pilot signals, sometimescalled pilot tones or simply pilots, are used to support channelestimation operations. Traditional channel estimation operations attemptto estimate the amplitude and phase distortion introduced by thecommunications channel and may thus compensate for the SNR lossesdiscussed above. Although more effective than many of the othertechniques, channel estimation operations are not precise enough tocompletely compensate for all SNR losses.

What is needed, therefore, are improved methods and systems for reducingPAPR OFDM based DVB-T system receivers that may compensate forcorresponding losses in SNR.

SUMMARY OF EMBODIMENTS

Consistent with the principles of the present invention as embodied andbroadly described herein, the present invention includes a method forperforming channel estimation in an Orthogonal Frequency DivisionMultiplexed (OFDM) signal. The method includes performing the channelestimation based upon use of reserved tone channel carriers.

Embodiments of the present invention provide reductions in PAPRcomparable to the traditional techniques above. In the embodiments ofthe present invention, however, SNR losses are compensated for byimproving the precision of the channel estimation process.

By way of example, OFDM systems work by splitting a wide-band digitalsignal into a large number of slower digital streams and thentransmitting all of the streams on a set of closely spaced adjacentcarrier frequencies (i.e., tones). Each of the streams includesdifferent types of tones. For example, a typical stream (e.g., symbol)might include data carriers, reserved tones, continuous pilots, andscattered pilots. As understood by those of skill in the art, some ofthese tones may be reserved for PAPR reduction. Once the data carriersare filled with data, appropriate values of the extra data (i.e.,channel estimation data) are inserted into the reserved carriers todecrease the PAPR of the time domain signal.

The present invention parameterizes an allowable set of values for thereserved tones. Typical parameterizations include values from a linearfeedback shift register, or a finite number of known quantized values.The transmitter finds the best signal from the parameterized set ofvalues for PAPR reduction, during an initialization process, and thensignals the chosen parameters (possibly in a robust channel). Thereceiver then decodes the parameters and uses the reserved tones asadditional pilots to improve channel estimation.

Some PAPR reduction is typically lost because the search space for datavalues for the reserved tones is reduced. However, given the moreaccurate channel estimation operations of the present invention,additional known techniques such as the Adaptive Constellation Extension(ACE) may be applied to gain further reductions. The process ofcomputing the final reserved tone values may be arrived at via aniterative technique that includes multiple PAPR reduction techniques.

A method for performing channel estimation in an orthogonal frequencydivision multiplexing (OFDM) signal includes choosing reserved tones tobe part of a pilot pattern, and using the reserved tones in the pilotpattern to perform the channel estimation.

An apparatus for use in performing channel estimation in an orthogonalfrequency division multiplexing (OFDM) system includes a receiver, apilot symbol extractor, and a channel estimator. The receiver isconfigured to receive a transmitted OFDM signal. The pilot symbolextractor is configured to extract pilot symbols from the OFDM signal.The channel estimator is configured to perform the channel estimation,including using reserved tones as pilot tones.

A non-transitory computer-readable storage medium stores a set ofinstructions for execution by a general purpose computer to perform amethod for providing channel estimation in an orthogonal frequencydivision multiplexing (OFDM) signal. The set of instructions includes achoosing code segment for choosing reserved tones to be part of a pilotpattern, and a using code segment for using the reserved tones in thepilot pattern to perform the channel estimation.

Further embodiments, features, and advantages of the present invention,as well as the structure and operation of the various embodiments of thepresent invention, are described in detail below with reference to theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated herein and form partof the specification, illustrate the present invention and, togetherwith the description, further serve to explain the principles of theinvention and to enable a person skilled in the relevant art(s) to makeand use the invention.

FIG. 1 is a block diagram illustration of a conventional OFDM basedDVB-T system;

FIG. 2 is a graphical illustration of an exemplary pilot patternsequence according to known channel estimation techniques;

FIG. 3 is a graphical illustration of a single symbol sequence beforechoosing reserved tone values according to known channel estimationtechniques;

FIG. 4 is a graphical illustration of a single symbol sequence beforechoosing reserved tone values according to known channel estimationtechniques;

FIG. 5 is a graphical illustration of a single symbol sequence afterchoosing reserved tone values from known pattern subsets in accordancewith an embodiment of the present invention; and

FIG. 6 is an exemplary flowchart of a method of practicing an embodimentof the present invention.

DETAILED DESCRIPTION

The following detailed description of the present invention refers tothe accompanying drawings that illustrate exemplary embodimentsconsistent with this invention. Other embodiments are possible, andmodifications may be made to the embodiments within the spirit and scopeof the invention. Therefore, the detailed description is not meant tolimit the invention. Rather, the scope of the invention is defined bythe appended claims.

It would be apparent to one of skill in the art that the presentinvention, as described below, may be implemented in many differentembodiments of software, hardware, firmware, and/or the entitiesillustrated in the figures. Any actual software code with thespecialized control of hardware to implement the present invention isnot limiting of the present invention. Thus, the operational behavior ofthe present invention will be described with the understanding thatmodifications and variations of the embodiments are possible, given thelevel of detail presented herein.

FIG. 1 is a block diagram showing constructions oftransmission/reception terminals of a conventional OFDM mobilecommunication system. Referring to FIG. 1, a mobile communication systemusing OFDM scheme includes a transmission terminal 100 and a receptionterminal 150. The transmission terminal 100 includes a data transmitter102, a coder 104, a symbol mapper 106, a serial to parallel (S/P)converter 108, a pilot symbol inserter 110, an inverse fast Fouriertransform (IFFT) unit 112, a parallel to serial (P/S) converter 114, aguard interval inserter 116, a digital-to-analog (D/A) converter 118,and a radio frequency (RF) processor 120.

In the transmission terminal 100, the data transmitter 102 generates andoutputs user data bits and control data bits to be transmitted to thecoder 104. The coder 104 receives and codes the signals outputted fromthe data transmitter 102 according to a predetermined coding scheme, andthen outputs the coded signals to the symbol mapper 106. The coder 104may perform coding by means of a convolutional coding scheme or a turbocoding scheme having a predetermined coding rate. The symbol mapper 106modulates the coded bits outputted from the coder 104 according to acorresponding modulation scheme, thereby generating modulation symbols,and outputs the modulation symbols to the S/P converter 108. Here, themodulation scheme that the symbol mapper 106 may follow includes, e.g.,a binary phase shift keying (BPSK) scheme, a quadrature phase shiftkeying (QPSK) scheme, a 16 quadrature amplitude modulation (QAM) scheme,64 QAM scheme, or others.

The S/P converter 108 receives and converts the serial modulationsymbols outputted from the symbol mapper 106 into parallel modulationsymbols, and outputs the converted parallel modulation symbols to thepilot symbol inserter 110. The pilot symbol inserter 110 inserts pilotsymbols into the converted parallel modulation symbols outputted fromthe S/P converter 108 and then outputs them to the IFFT unit 112.

The IFFT unit 112 receives the signals outputted from the pilot symbolinserter 110, performs N-point IFFT for the signals, and then outputsthem to the P/S converter 114. The P/S converter 114 receives thesignals outputted from the IFFT unit 112, converts the signals intoserial signals, and outputs the converted serial signals to the guardinterval inserter 116. The guard interval inserter 116 receives thesignals outputted from the P/S converter 114, inserts guard intervalsinto the received signals, and then outputs them to the D/A converter118. Here, the inserted guard interval prevents interference betweenOFDM symbols transmitted in the OFDM communication system; that is tosay, the inserted guard interval prevents interference between aprevious OFDM symbol transmitted during a previous OFDM symbol periodand a current OFDM symbol to be transmitted during a current OFDM symbolperiod.

In inserting the guard interval, a method of inserting null data hasbeen proposed. However, insertion of null data as a guard interval maycause a receiver to erroneously estimate a start point of an OFDMsymbol, thereby allowing interference between sub-carriers, whichincreases the probability of erroneous determination of the startingpoint for received OFDM symbols. Therefore, a cyclic prefix method or acyclic postfix method is usually used in inserting the guard interval.In the cyclic prefix method, a predetermined number of last bits of anOFDM symbol are copied and inserted into an OFDM symbol. In the cyclicpostfix method, a predetermined number of initial bits of an OFDM symbolare copied and inserted into an OFDM symbol.

The D/A converter 118 receives the signals outputted from the guardinterval inserter 116, converts the signals into analog signals, andoutputs the converted analog signals to the RF processor 120. The RFprocessor 120 includes a filter and a front end unit. The RF processor120 receives the signals from the D/A converter 118, RF-processes thesignals, and then transmits the signals over the air through a transmitantenna. Hereinafter, the reception terminal 150 will be described.

The reception terminal 150 includes an RF processor 152, ananalog-to-digital (A/D) converter 154, a guard interval remover 156, aS/P converter 158, a fast Fourier transform (FFT) unit 160, a pilotsymbol extractor 162, a channel estimator 164, an equalizer 166, a P/Sconverter 168, a symbol demapper 170, a decoder 172, and a data receiver174.

The signals transmitted from the transmission terminal 100 pass throughmulti-path channels and are received by a receive antenna of thereception terminal 150 in a state in which noise is included in thesignals. The signals received through the receive antenna are inputtedto the RF processor 152, and the RF processor 152 down-converts thereceived signals into signals of an intermediate frequency (IF) band,and then outputs the IF signals to the A/D converter 154. The A/Dconverter 154 converts the analog signals outputted from the RFprocessor 152 into digital signals and then outputs the digital signalsto the guard interval remover 156.

The guard interval remover 156 receives the digital signals converted byand outputted from the A/D converter 154, eliminates guard intervalsfrom the digital signals, and then outputs them to the S/P converter158. The S/P converter 158 receives the serial signals outputted fromthe guard interval remover 156, converts the serial signals intoparallel signals, and then outputs the parallel signals to the FFT unit160. The FFT unit 160 performs N-point FFT on the signals outputted fromthe P/S converter 158, and then outputs them to both the equalizer 166and the pilot symbol extractor 162. The equalizer 166 receives thesignals from the FFT unit 160, channel-equalizes the signals, and thenoutputs the channel-equalized signals to the P/S converter 168. The P/Sconverter 168 receives the parallel signals outputted from the equalizer166, converts the parallel signals into serial signals, and then outputsthe converted serial signals to the symbol demapper 170.

As indicated, the signals outputted from the FFT unit 160 are alsoinputted to the pilot symbol extractor 162. The pilot symbol extractor162 detects pilot symbols from the signals outputted from the FFT unit160 and outputs the detected pilot symbols to the channel estimator 164.The channel estimator 164 performs channel estimation using the pilotsymbols and outputs the result of the channel estimation to theequalizer 166. Here, the reception terminal 150 generates channelquality information (hereinafter, referred to as “CQI”) corresponding tothe result of the channel estimation and transmits the CQI to thetransmission terminal 100 through a CQI transmitter (not shown).

The symbol demapper 170 receives the signals outputted from the P/Sconverter 168, demodulates the signals according to a demodulationscheme corresponding to the modulation scheme of the transmissionterminal 100, and then outputs the demodulated signals to the decoder172. The decoder 172 decodes the signals from the symbol demapper 170according to a decoding scheme corresponding to the coding scheme of thetransmission terminal 100 and outputs the decoded signals to the datareceiver 174.

In OFDM systems, data is transmitted and received via multiple carrierfrequencies. In some OFDM systems, for example, there may beapproximately 128 independent OFDM sub-carriers (i.e., tones) thatoccupy approximately 528 megahertz of bandwidth. In these systems, datais modulated and transmitted via many of these sub-carriers.Approximately 10 of the sub-carriers do not necessarily carryinformation. These sub-carriers, also referred to as pilot tones, may beused to guard the information carrying sub-carriers, to simplify thefiltering requirements of the system, or to provide referencephase/amplitude information for the demodulator. The positions of thepilot tones may be defined according to the communication standard ordefined by the user/designer of the system. Some pilot tones are locatedon two ends of the frequency spectrum and some of the pilot tones areinterspersed within the frequency spectrum.

For each transmitted carrier signal, an OFDM receiver normally attemptsto compensate for the distortion induced by the transmission channel.This will normally involve a channel estimation operation and a channelcompensation operation. To assist a receiver in overcoming multipathdistortion, pilot signals with known data patterns are transmitted. Thepilot signals, sometimes called pilot tones or simply pilots, are usedto support channel estimation operations. Such channel estimationoperations normally attempt to estimate the amplitude and phasedistortion introduced by the communications channel.

The pattern structure of the pilots may be in essentially any manner,provided that the Nyquist sampling criteria for the communicationchannel's impulse response and rate of change are satisfied. The numberof pilots transmitted is often a function of the expected multipathdistortion delay and the anticipated rate of change in channelconditions. However, for purposes of efficiency, it is desirable tominimize the number of pilots transmitted since the transmission of apilot precludes the transmission of data in the transmission slot usedto transmit the pilot.

DVB-T OFDM systems often use their pilot tones for the purpose of makingchannel estimation easier. However, the sparseness of these tonesrenders it difficult to estimate the channel quickly and with efficientmemory usage and calculations.

Channel estimation is an important and necessary function for modernwireless receivers. With even a limited knowledge of the wirelesschannel properties, a receiver may gain insight into the informationthat was sent by the transmitter. The goal of channel estimation is tomeasure the effects of the channel on known, or partially knowntransmissions. A channel will change with changing conditions andtopology. OFDM systems are especially suited for channel estimation. Thesubcarriers are closely spaced and the system is generally used in highspeed applications that are capable of computing channel estimates withminimal delay. Subcarriers that are sent with a known power and make-upare called pilots and are used for synchronization. As noted above, thepresent invention provides a unique and novel approach to accomplishingchannel estimation.

More specifically, the present invention provides enhancements to knownchannel estimation techniques, such as tone reservation. In tonereservation, for example, carriers are reserved and populated witharbitrary values in order to decrease the PAPR.

The present invention, on the other hand, is illustrated in thefollowing example. Assume that non-arbitrary values may be chosen forcarriers that are known. That is, for example, in a given symbol,carriers 3, 7, and 10 are available and instead of using arbitraryvalues for each of these carriers may take five values (e.g., −2, −1, 0,1, and 2). Thus, there are 125 choices (i.e., 5³). Assume that each ofthe 125 choices includes sufficient granularity such that the PAPR couldbe decreased by an amount comparable to PAPR reduction using thearbitrary values, noted above. In this scenario, the PAPR could bereduced almost as much as it could by using the arbitrary values. Byusing small sets of non-arbitrary values, however, the receiver has moreinformation with which to perform channel estimates and equalization.The ability to provide this information to the receiver enables thesecarriers to be used as pilots. Traditionally, only dedicated pilotscould be used to provide channel estimates. The present inventionenables additional, non-pilot channel carriers, to be used to providethe channel estimates.

FIG. 2 is a graphical illustration of a pilot pattern sequence 200formulated in accordance with known channel estimation techniques. Thepilot pattern sequence 200 includes a symbol group 202 of nine OFDMsymbols arranged along a Y axis 204. The Y axis 204, for example, may bein gradations of 200 microseconds (μs) up to 1 millisecond. The fidelityof these gradations is dependent upon the total duration of each of theOFDM symbols. The graph of FIG. 2 also includes an X axis 206.

Each of the symbols within the symbol group 202 includes differentcarrier types comprising data carriers (d_(i,j)), continuous pilots(c_(i,j)), scattered pilots (s_(i,j)), and reserved tones (r_(i,j)). Thereserved tones (r_(i,j)) are generally unused. Although the pilotsequence 200 is representative of a sequence configured for use in aDVB-T2 system, it may apply to any OFDM system.

The first element in the carrier type subscript (e.g. d_(i,j))represents a carrier index. The second element of the subscriptrepresents a time index. For example, in FIG. 2, a first OFDM symbol207, occurring at time 0 includes a continuous pilot c_(0,0), a datacarrier d_(1,0), a data carrier d_(2,0), etc.

In an OFDM symbol, the data carriers are representative of actualtransmitted data. The continuous pilots and the scattered pilots areprovided to be able to perform channel estimation. For the symbol 207,the continuous pilots are c_(0,0) and c_(15,0). The symbol 207 alsoincludes a scattered pilot s_(12,0). The notion of why two differenttypes of pilots are used is well understood by those of skill in the artand will not be discussed herein.

Focusing on other aspects of pilots signals, most OFDM systems includethe concept of having pilots that are not located within any particularsystem. For example, in the symbol group 202, all of the continuouspilots have a carrier index identical to other carrier pilots, meaningthey are in vertical columns with other continuous pilots. For example,there are continuous pilots at carrier index “0” and carrier index “15.”The fact that the continuous pilots are all in vertical columns togethermeans that the pilot location stays the same, across different symbols.That is, if carrier 0 is a pilot in symbol 0 (i.e., symbol 207), thencarrier 0 is also a pilot in a symbol 208, carrier 0 is also a pilot insymbol 210, etc. This process is typical of DVB-T systems and wirelesslocal area network (LAN) systems.

The problem, however, with pilot configurations such as theconfiguration of FIG. 2 is that the density of the continuous pilots istypically insufficient to adequately perform channel estimation. Tocompensate for this lack of continuous pilot density, roving or“scattered” pilots are provided to augment the channel estimationcapability of the continuous pilots. In FIG. 2, as discussed above, inthe symbol 207 the scattered pilot is denoted as s_(12,0). The symbol208 includes a scattered pilot s_(9,1), etc. These are the typical typesof pilots that may be found in conventional OFDM systems.

FIG. 3 is a graphical illustration 300 of a single symbol sequence,prior to choosing reserved tone values, according to known channelestimation techniques. More specifically, FIG. 3 is an illustration ofthe single symbol 207 shown in FIG. 2, and is representative of a singlesymbol occurring at time index “0.” In FIG. 3, the “0s” have beenremoved from the time index column for purposes of simplification.

By way of background, before reserved tones, such as the reserved tones(r_(i,j)), are assigned proper non-zero values, the data carriers(d_(i,j)), the continuous pilots (c_(i,j)), and the scattered pilots(s_(i,j)j) are assigned specific values first. After this assignment, afast Fourier transform (FFT) 302 is applied to the values assigned tothe data carriers, the continuous pilots, and the scattered pilots tocreate a time domain signal 304. This newly computed time domain signal304, for example, might potentially have a high PAPR. If the time domainsignal 304 has a high PAPR and if there are no reserved tones, thecontinuous pilots (c_(i,j)), and the scattered pilots (s_(i,j)) could beused to perform the channel estimation to decode (and record) the dataembedded within the data carriers (d_(i,j)). In FIG. 3, values for thereserved tones (r₅) and (r₁₉) are chosen as zero.

FIG. 4 is a graphical illustration 400 of the single symbol 207 beforechoosing reserved tone values according to known channel estimationtechniques. In FIG. 4, arbitrary values for the reserved tones (r_(i,j))are selected. By way of example, using methods well understood by thoseof skill in the art, arbitrary non-zero complex values for the reservedtones (r₅) and (r₁₉) may be selected. Methods for reserve tonedetermination and selection are provided, for example, in a Thesis by J.Tellado, Peak to average power reduction for multicarrier modulation,Ph.D. dissertation, Stanford, Calif.: Stanford Univ., 2000. In FIG. 3,the values for (r₅) and (r₁₉) are zero.

In FIG. 4, these reserve tone values, although arbitrary, are chosen tobe non-zero values. More specifically, the non-zero values are chosensuch that when an FFT 402 is applied thereto, a minimum PAPR may beachieved since the continuous pilots (c_(i,j)), the scattered pilots(s_(i,j)) and the data carriers (d_(i,j)) are fixed.

Before application of reserved tones, all carriers must have some value.Since the maximum transmit power is limited by regulation, if no carrieris used, its value is set to 0 so that the useful carriers may betransmitted at higher power. For the purposes of the present invention,however, these values may be selected randomly. This random selectionwould cause, with very high probability, the equivalent time domainsignal to have high PAPR.

In embodiments of the present invention, instead of choosing thereserved tone values (r₅) and (r₁₉) as arbitrary values, they are chosento be part of a pilot pattern from some limited set, which is from asubset of reserved carrier pilots. In the present invention, thereserved tone values are not the same as the arbitrary values from FIG.4. Instead, the reserved tone values (r₅) and (r₁₉) are chosen to beclose enough to the reserved tone values from FIG. 4 such that theresulting signal has a low PAPR. As a result of this value selectionprocess, the reserved tones (r₅) and (r₁₉) may be used as pilots toassist and improve the channel estimation process. This process isillustrated more clearly in FIGS. 5 and 6 below.

In accordance with known methods, such as those highlighted in thethesis noted above, reserved tone values may be determined. All of thesevalues may be determined ahead of time. They may also, however, bedetermined dynamically during an iterative process that commences duringreceiver initialization. Generally, all these methods use a projectionof signal peaks onto the reserved tones to produce a reduction signal.This reduction signal is then used for peak-canceling purposes in orderto lower the PAPR of the OFDM transmitter.

In a first method, the first k reserved tones are allowed to be any ofthe elements from a known large set such as {a+ib, a is a multiple of0.08 and so is b}. The later reserved tones are produced by filteringthe previous tones with a known filter. In this scenario, the receiverwould estimate the first k reserved tones using traditional methods andthe rest would follow from them. Allowing for 64 different filters(requiring only 6 bits of signaling), allowing a and b to be between −2and +2, and allowing k to be a small number (e.g., 15), numerouspossible tone patterns may provide substantial PAPR reductions.

A second method for choosing reserved tone values is essentially thesame as the first method above. In the second method, however, a and bare generated using linear feedback shift registers (LFSRs). Numerousother well known techniques for choosing reserve tone values areavailable.

FIG. 5 is a graphical illustration 500 of the single symbol sequence 207after choosing reserved tone values from known pattern subsets, inaccordance with an embodiment of the present invention. In FIG. 5, forexample, in order to decrease the PAPR, the reserved tones (r₅) and(r₁₉), which have similar characteristics to continuous pilots (c₀, andc₁₅) and scattered pilot s₁₂, are used to assist in enhanced channelestimation. Although specifically dedicated reserve tones are used inthe example of FIG. 5, scattered pilots could be used instead ofreserved tones.

In FIG. 5, the value of reserved tones (r₅) and (r₁₉) may be determinedusing one of the methods noted above. More specifically, reserved tonevalues for (r₅) and (r₁₉), shown as (r′₅) and (r′₁₉) respectively, arechosen to be non-zero and non-arbitrary. An FFT 502 is applied to thevalues (r′₅) and (r′₁₉) to produce a low, but not necessarily a minimum,PAPR 504.

The present invention includes embodiments that transmit known values toan OFDM receiver using reserved tones to provide more precise channelestimation. Middle values, chosen for the reserved tones, may be addedor subtracted in accordance with the technique illustrated in FIG. 5,for all of the carriers. Middle values are those values that may beconstrained (i.e., adding or subtracting other values) in different waysthat will not adversely affect the PAPR reduction. These values may beconstrained in some way, as represented by constraint sets A5 and A19shown in FIG. 5, and are derived from a finite set. In this manner, theOFDM receiver has additional information that may be used to providemore accurate channel estimates, or decode or de-map the actualtransmitted value more accurately. This process may also be used fortone injection.

In the embodiments discussed above, the use of middle values allowsvarious techniques to be used to decrease the PAPR while simultaneouslypermitting reserved tone carriers, for example, r′₅ and r′₁₉, shown inFIG. 5, to be used as pilot tones. If arbitrary complex values are usedfor the reserved tone carriers, higher PAPR reduction values may beachieved. However, it is possible to use sub-optimal, albeit acceptable,values for PAPR reduction, where there may be two or three possibleconstraint set solutions. For example, if three pilot carriers are used,middle values could be chosen such as 1, −1, and 2. Many othercombinations of middle values may be chosen. So with these middlevalues, sequences may be used that will not adversely affect the PAPRreduction capabilities, but would fit within the constraint sets A5 andA19 (i.e., the set of values actually permitted), associated with r′₅and r′.₁₉, respectively.

FIG. 6 is an exemplary flowchart 600 of a method of practicing anembodiment of the present invention. More specifically, the method 600includes a method for decreasing PAPR for a multicarrier modulationsignal including two or more groups of carriers. In a step 602 during afirst timeframe, non-zero values are assigned for carrier types within afirst of the two or more groups. Additionally, zero values are assignedfor carrier types within a second of the two or more groups afterassigning the non-zero values. During a second timeframe in step 604,arbitrary non-zero values are chosen for the carrier types within thesecond group. Within a step 606 during a third timeframe, non-zerovalues for the carrier types within the second group are chosen basedupon a predetermined PAPR threshold. The method 600 may be implemented,for example, during a transmitter training phase or an initializationprocess.

Conclusion

It is to be appreciated that the Detailed Description section, and notthe Summary and Abstract sections, is intended to be used to interpretthe claims. The Summary and Abstract sections may set forth one or morebut not all exemplary embodiments of the present invention ascontemplated by the inventor(s), and thus, are not intended to limit thepresent invention and the appended claims in any way.

The present invention has been described above with the aid offunctional building blocks illustrating the implementation of specifiedfunctions and relationships thereof. The boundaries of these functionalbuilding blocks have been arbitrarily defined herein for the convenienceof the description. Alternate boundaries may be defined so long as thespecified functions and relationships thereof are appropriatelyperformed.

The foregoing description of the specific embodiments will so fullyreveal the general nature of the invention that others may, by applyingknowledge within the skill of the art, readily modify and/or adapt forvarious applications such specific embodiments, without undueexperimentation, without departing from the general concept of thepresent invention. Therefore, such adaptations and modifications areintended to be within the meaning and range of equivalents of thedisclosed embodiments, based on the teaching and guidance presentedherein. It is to be understood that the phraseology or terminologyherein is for the purpose of description and not of limitation, suchthat the terminology or phraseology of the present specification is tobe interpreted by the skilled artisan in light of the teachings andguidance.

The breadth and scope of the present invention should not be limited byany of the above-described exemplary embodiments, but should be definedonly in accordance with the following claims and their equivalents.

The claims in the instant application are different than those of theparent application or other related applications. The Applicanttherefore rescinds any disclaimer of claim scope made in the parentapplication or any predecessor application in relation to the instantapplication. The Examiner is therefore advised that any such previousdisclaimer and the cited references that it was made to avoid, may needto be revisited. Further, the Examiner is also reminded that anydisclaimer made in the instant application should not be read into oragainst the parent application.

What is claimed is:
 1. A method for performing channel estimation on anorthogonal frequency division multiplexing (OFDM) signal, comprising:choosing reserved tones to be part of a pilot pattern, wherein thechoosing includes: using a projection of signal peaks onto the reservedtones to produce a reduction signal; and using the reduction signal forpeak canceling to lower a peak to average power ratio; and using thereserved tones in the pilot pattern to perform the channel estimation.2. The method according to claim 1, wherein the choosing includes usingpredetermined reserved tone values.
 3. The method according to claim 1,wherein the choosing includes dynamically determining reserved tonevalues.
 4. The method according to claim 3, wherein the reserved tonevalues are dynamically determined via an iterative process during areceiver initialization process.
 5. The method according to claim 1,wherein the channel estimation is performed in a receiver configured toreceive the OFDM signal.
 6. An apparatus for use in performing channelestimation on an orthogonal frequency division multiplexing (OFDM)system, comprising: a receiver configured to receive a transmitted OFDMsignal; a pilot symbol extractor configured to extract pilot symbolsfrom the OFDM signal; and a channel estimator configured to: choosereserved tones to be part of a pilot pattern; use a projection of signalpeaks onto the reserved tones to produce a reduction signal; use thereduction signal for peak canceling to lower a peak to average powerratio; and use the reserved tones in the pilot pattern to perform thechannel estimation.
 7. A non-transitory computer-readable storage mediumstoring a set of instructions for execution by a general purposecomputer to perform a method for providing channel estimation on anorthogonal frequency division multiplexing (OFDM) signal, the set ofinstructions comprising: a choosing code segment for choosing reservedtones to be part of a pilot pattern, wherein the choosing code segmentincludes: a first using code segment for using a projection of signalpeaks onto the reserved tones to produce a reduction signal; and asecond using code segment for using the reduction signal for peakcanceling to lower a peak to average power ratio; and a third using codesegment for using the reserved tones in the pilot pattern to perform thechannel estimation.
 8. The computer readable medium according to claim7, wherein the choosing code segment includes using predeterminedreserved tone values.
 9. The computer readable medium according to claim7, wherein the choosing code segment includes dynamically determiningreserved tone values.
 10. The computer readable medium according toclaim 9, wherein the choosing code segment further includes dynamicallydetermining the reserved tone values via an iterative process during areceiver initialization process.